1. Field of the Invention
The present invention relates to a lamp circuit having two or more lamps connected in series and more particularly, to a lamp circuit providing means for detecting a disconnected lamp.
2. Description of the Prior Art
One way of using of a series lamp circuit is for lighting an airport runway. A series lamp circuit includes two or more current transformers, each secondary winding of which is connected to the corresponding lamp and each primary winding of which is connected in series with each other. Those lamps are supplied by a constant current source through current transformers having primary windings connected in series so that the lamps are all lit at constant luminance.
The series lamp circuit provides a disconnected lamp detecting device which is capable of detecting a disconnected lamp on a power operation panel. A lamp circuit having such a series lamp circuit has been proposed in U.S. Pat. No. 4,295,079 and 4,396,868, for example.
These prior art, however, have no capability of detecting the location of a disconnected lamp(s) in the overall lamp system, though they are capable of detecting occurrence of a disconnected lamp and the number of disconnected lamps.
In order to overcome this shortcoming, there has been proposed a technique having a capability of detecting the number of disconnected lamps and the location of the disconnected lamps. This technique has been disclosed in Japanese Patent Application Laid Open No. Heisei 1-335292 (335292/1989).
In the disclosed art, the lamps respectively provide corresponding terminal units and are all connected to a common master station located on the power operation panel. Between each corresponding unit and the master station are provided load current circuits. The load current circuit is capable of detecting the number of disconnected lamps as well as the locations of the disconnected lamps on the power operation panel. This prior art, therefore, enables quickly replacing the disconnected lamp with a new one.
If a disconnected lamp occurs, however, an excess voltage appears in the secondary of the transformer connected with the lamp. The excess voltage often causes parts included in the corresponding terminal unit to fail. The failure results in disabling to carry out the essential function of the terminal unit.